The present invention relates to a trackbed liner. In particular, but not exclusively, the present invention relates to a trackbed liner having a filtration layer which restricts the passage of solids materials through the liner. The present invention also relates to a method of manufacturing such a trackbed liner, and to methods of renovating a trackbed, of laying a track, and of displacing water from a trackbed utilising such a trackbed liner.
In the field of railway track construction, a railway is formed by constructing a foundation known as a trackbed on to which a railway track is laid. The trackbed typically comprises a 300 to 500 mm depth layer of ballast of a graded, crushed rock aggregate. The ballast is laid on a naturally occurring soil formation known as a subgrade in the region where the railway is to be constructed. As is well known, in areas where the subgrade is clay, or of a soil having a high clay content, a problem known as “pumping erosion” can occur. Pumping erosion is where, under the load of a passing train, naturally occurring groundwater is urged out of the subgrade and into the ballast. The groundwater carries fine clay particles which are deposited in the ballast and, over time, this causes a significant erosion of the subgrade, leading to settlement of the track. Additionally, migration of clay particles into the ballast has the effect of reducing friction between the aggregate materials in the ballast layer. This can result in sleepers on which rails of the track are laid settling into the ballast.
Pumping erosion is a particular problem in the railway industry due to the large, high frequency cyclical loads imparted on the trackbed during passage of a fast-moving train along the track. These loads are transmitted from the train wheels down through rails and sleepers forming the track and into the ballast, and thus to the subgrade. Also, passage of the train along the tracks can create a wave which travels along the trackbed, enhancing the pumping erosion effect.
Traditionally, this problem has been addressed by providing a layer (typically 100 to 150 mm deep) of sand on the subgrade beneath the ballast. The sand acts as a natural filter, slowing passage of water between the subgrade and the ballast, but trapping fine clay particles. Whilst this has been found to be an effective solution to the problem, the additional bulk material, transportation and labour costs involved add significantly to the overall construction cost. It is therefore generally desired to provide an alternative solution to the problem of pumping erosion.
In recent years, attempts have been made to find a textile-based solution to the problem. International patent application number PCT/GB2007/002502 (published as WO-2008/009882) to WTB Group Limited and Terram Limited discloses a containment structure comprising an open-cell matrix, and an intermediary composite comprising particulate material retained in a support matrix. The particulate material is typically sand. However, the proposed solution does not fully address the problem, and the containment structure remains bulky and heavy and thus expensive to store, transport and install. Additionally, large volumes of sand are still employed, with associated bulk material costs, and the manufacturing process, involving charging of the sand into an open-cell matrix of the containment structure, is not straightforward.
It will be understood that similar problems may occur in other, related industries such as in tramway track construction.
It is amongst the objects of embodiments of the present invention to obviate or mitigate at least one of foregoing disadvantages.